A simple UNIX client allowing you to send SMS messages to mobile phones
and pagers. The software currently supports a number of providers
and protocols from various countries.

Using an unlisted provider that allows TAP access should be pretty straight
forward.

There are a large number of services that do not appear to use TAP, but
instead, simple user interfaces for interactive use by a user dialing up
with a modem. For several UK based services such as these drivers have been
written, note that providers often offer more that one service and as such
you may require a different driver for each one.

WWW: https://bsdforge.com/projects/comms/sms_client/